It is legal to sell designer inspired, sometimes called replicas, however it is illegal to sale fakes, also sometimes referred to as replicas, so you need to really be switched on when getting into this market.
What's the difference?, well the biggest difference between a designer inspired replica and a straight out fake replica, is branding. A non-authentic product can NOT carry a brand that they are not the legal owner of.
There may also be design copyright infringements to be concerned about as well, it can be a bit of a mine filed if you're not 100% sure.
In regards to packaging, you have to bare in mind that you are dealing with a breakable item, and as with most postal services in the world, if it's breakable, they will usually find a way to break it, so hard cover packaging would certainly be advised in my opinion.